Which type of natural selection increases the frequency of one extreme phenotype?

Directional Selection

Explanation:

Directional selection is a type of natural selection that occurs when individuals of an extreme phenotype possess greater survival chances compared to the other individuals. With time, the individuals of the extreme phenotype become more established than the other individuals of the other extreme phenotype having lower fitness survive. As a result, the frequency of the extreme phenotype that has greater fitness increases while the other decreases.